Catalysts for purification of exhaust gas have been conventionally used for removing harmful components such as hydrocarbon gas (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in exhaust gas from automobile engines. As such catalysts for purification of exhaust gas, three-way catalysts are known that simultaneously clean HC, CO and NOx in exhaust gas combusted in the theoretical air-fuel ratio and are generally composed of: a substrate (support substrate) made of cordierite, a metal foil, or the like, and formed in a honeycomb shape; a support (catalyst supporting layer) made of an active alumina powder, a silica powder, or the like, and formed on the substrate surface; and a catalyst component made of a noble metal such as platinum, and supported on the support.
For example, Japanese Unexamined Patent Application Publication No. Hei 5-317652 (Document 1) discloses a catalyst for purification of exhaust gas produced by supporting an alkaline earth metal oxide and platinum on a support formed of a porous body. In addition, Japanese Unexamined Patent Application Publication No. Hei 6-99069 (Document 2) discloses a catalyst for purification of exhaust gas comprising a support substrate and a catalyst component layer produced by supporting, on the surface of the support substrate, catalyst components of 1 g to 20 g of palladium, of 50 g to 250 g of alumina, of 10 g to 150 g of cerium oxide, and of 8 g to 50 g of barium oxide per liter of the support substrate volume. Additionally, Japanese Unexamined Patent Application Publication No. Hei 10-174866 (Document 3) discloses a catalyst for purification of exhaust gas comprising a first catalyst layer produced by supporting at least palladium on a first porous support, and a second catalyst layer formed on the surface of the first catalyst layer and produced by supporting at least rhodium on a second porous support, wherein the mass of the palladium, supported on the first catalyst layer, per unit mass of the first porous support is larger than the mass of the rhodium, supported on the second catalyst layer, per unit mass of the second porous support.
However, when the catalysts for purification of exhaust gas as described in Documents 1 to 3 is exposed to high-temperature exhaust gas (particularly, 800° C. or higher), such problem occurs that catalytic activity will be lowered because particles of a noble metal having catalytic activity such as platinum, rhodium or palladium supported on its support are aggregated and then sintering (particle growth) occurs to decrease the specific surface area.
Furthermore, Japanese Unexamined Patent Application Publication No. 2004-41866 (Document 4) discloses a catalyst for purification of exhaust gas comprising a composite oxide of a perovskite structure expressed by a specific formula, the composite oxide including: at least one element selected from rare earth elements that surely contain a rare earth element and do not contain a rare earth element capable of having a valence smaller than 3; at least one element selected from A1 and transition elements except cobalt, palladium and the rare earth elements; and palladium. However, in the catalyst for purification of exhaust gas as described in Document 4, a noble metal is solid-dissolved in a perovskite structure and is stable in an oxidation state, thereby posing the problem that the noble metal contained in its structure has a difficulty in functioning as the active site of a catalyst, so that the catalytic activity is not still sufficient.
Moreover, Japanese Unexamined Patent Application Publication No. 2003-220336 (Document 5) discloses a catalyst for purification of exhaust gas comprising a support containing cerium oxide and a catalyst metal including a transition metal and a noble metal and supported on at least the cerium oxide, wherein the relationship between the atomic ratio of the transition metal to the cerium atom and the atomic ratio of the transition metal to the noble metal falls in a specified range. However, the catalyst for purification of exhaust gas as described in Document 5 is not still sufficient in that the catalytic activity is regenerated by re-dispersing a noble metal by regeneration processing.
In addition, Japanese Unexamined Patent Application Publication No. 2005-270882 (Document 6) discloses a catalyst produced by supporting one kind, or two kinds or more, of catalyst metal particles including one kind, or two kinds or more, of transition metals or transition metal oxides of an atomic number of from 10 to 50,000 on a porous support including oxides of one kind, or two kinds or more, of ceria, ceria-zirconia, ceria-zirconia-yttria and ceria-lanthanum-zirconia. Moreover, Japanese Unexamined Patent Application Publication No. 2002-79053 (Document 7) discloses a catalyst for purification of exhaust gas produced by coating a fire-proof three-dimensional structure with a catalyst active component containing zirconium oxide composition that contains at least one kind of noble metals, fire-proof inorganic oxide, cerium and lanthanum and has a single structure of zirconium oxide having a crystal structure of a tetragonal form. Furthermore, Japanese Unexamined Patent Application Publication No. 2004-141833 (Document 8) discloses a catalyst for purification of exhaust gas, in which a noble metal is supported on metal oxide particles containing ceria and zirconia, and the metal oxide particles have a central portion containing more ceria than zirconia and a surface layer containing more zircinia than ceria. In addition, Japanese Unexamined Patent Application Publication No. 2004-243177 (Document 9) discloses a catalyst for purification of exhaust gas, in which a noble metal is supported on composite oxide particles containing at least CeO2 and ZrO2 in each particle, in which the relation 0.5≦CZr/CCe≦1.5 is satisfied when CCe represents % by weight of CeO2 of the composite oxide powder and CZr represents the % by weight of ZrO2 of the composite oxide powder, and in which the noble metal is supported on the composite oxide powder by using an aqueous noble metal salt solution having a pH value lower than that of a suspension produced by immersing the composite oxide powder in pure water.
However, in the catalyst as described in Document 6, because a noble metal is supported as a cluster to thereby attempt thermal stabilization of the noble metal particles, there is a problem that, if a noble metal resistant to a higher temperature is used, the catalytic activity per unit amount of the noble metal is lowered. Additionally, in a catalyst for purification of exhaust gas as described in Document 7, the number of noble metal holding sites is insufficient, thereby posing the problem of the noble metal grain-growing to lower the catalytic activity. Moreover, since the catalysts for purification of exhaust gas as described in Documents 8 and 9 are not uniform in the compositions of cerium and zirconium within the support particles, the heat-proof is inferior, and therefore they have insufficient inhibition of the grain growth of the noble metal. Furthermore, in the catalysts for purification of exhaust gas as described in Document 6 to 9, the catalytic activity per unit amount of a noble metal after use for a long period of time is insufficient and sufficient catalytic activity is not regenerated by regeneration processing.
On the other hand, for solving the problem of the catalytic activity being lowered by sintering as described above, various methods for regenerating catalysts for purification of exhaust gas in which grain growth is generated in noble metal particles have been developed. For instance, Japanese Unexamined Patent Application Publication No. Hei 7-75737 (Document 10) discloses a method for regenerating a catalyst for purification of exhaust gas produced by supporting a noble metal as an active species in an inorganic porous matrix that contains causing halogen to act on the catalyst to generate a halide of a noble metal on the matrix and then removing the halogen from the halide. However, in the method of regenerating a catalyst for purification of exhaust gas, in which method halogen is caused to act on the catalyst as described in Document 10, regeneration of the catalyst is very difficult in a state in which the catalyst is installed in an exhaust system of an internal combustion engine, and there is a limit on shortening of the time needed for regeneration processing by re-dispersing the gain grown noble metal to regenerate the catalytic activity.
In addition, Japanese Unexamined Patent Application Publication No. 2000-202309 (Document 11) discloses a method in which oxidation treatment is applied to a catalyst for purification of exhaust gas comprising a support containing at least one species selected from alkaline earth metal oxides and rare earth oxides, and platinum supported on the support, and subsequently reduction treatment is applied to the catalyst. However, even the method described in Document 11 is insufficient from the viewpoint of the shortening of time and reducing of temperature, needed for regeneration processing by re-dispersing gain grown platinum particles to regenerate the catalytic activity.